This invention relates to steel sheet plated with a Zn-Ni alloy with improved adhesion upon impact as well as improved resistance to powdering while exhibiting improved resistance to corrosion. In particular, the invention relates to a steel sheet electroplated with a Zn-Ni alloy which is especially advantageous when used for outer panes of automobiles.
Recently, there is an increasingly strong demand for automobile bodies having improved corrosion resistance. The so-called "10-5" guideline to the corrosion resistance has been announced for accelerating development of electroplated steel sheet for use in automobiles. It is extremely important to satisfy this guideline. The guideline "10-5" stands for "No perforation for 10 years, and no cosmetic corrosion for 5 years". These two requirements are considered most important, and various types of precoated steel sheets have been developed or proposed to satisfy them. Among these electroplated steel sheets, Zn-Ni alloy-electroplated steel sheets have the best overall characteristics including their improved corrosion resistance and paintability. Due to their superior resistance to perforation corrosion, Zn-Ni alloy-electroplated steel sheets have been used for the inner panels of automobiles.
Generally, panels for automobiles must exhibit good adhesion to paint coatings. Therefore, it is common for an outer panel of an automobile to be disposed with its cold rolled surface facing outwards because the cold-rolled surface is good for paint coating. When an electroplated steel sheet is employed for an outer pane, only one side of the sheet is electroplated and the unplated surface is made to face outwards. However, steel sheet which is electroplated on both sides is becoming increasingly common since the exterior outer panels also needs to be highly corrosion resistant.
The Zn-Ni alloy electroplated steel sheet has the following defects which must be overcome before it can be satisfactorily employed for the outer panels of modern automobiles.
(i) The plating layer of conventional Zn-Ni alloy electroplated steel sheet contains 10-16% by weight of Ni. This level of N is necessary to achieve a satisfactory level of corrosion resistance. However, the plating layer comprises hard intermetallic compounds of a single .gamma.-phase. Therefore, when the steel sheet is used for the outer panels of automobiles, peeling of the painting layer and even the plating layer can easily occur due to impact with gravel, which often strikes against the outer panels. This peeling under impact is caed "chipping". The precoated steel sheet easily undergoes red rusting in the area where the plating is peeled off. This is an extremely serious drawback, since it is greatly desired the surface of an exterior outer panel of an automobile be kept free of rust.
(ii) Steel sheet is shaped into automobile outer panels by pressing. During press-forming, the hard plating layer of a singe .gamma. phase is easily cracked, and the cracked plating layer easily peels off the steel substrate during sliding on the press die, resulting in a degradation in the corrosion resistance without paint.
Thus, in order for conventional Zn-Ni alloy electroplated steel sheet to be used for automobile outer panels, the adhesion of an electroplated layer to the substrate when an impact is applied (hereunder referred to as "impact adhesion"or "adhesion upon impact") and the adhesion of the electroplated layer to the substrate during pressing (hereunder referred to as "adhesion after processing" or "adhesion after forming") must be improved. Some proposals for improving these properties are as follows:
.circle.1 U.S. Pat. No.3,558,442 specifies certain plating bath conditions including composition, pH. and temperature, as well as the electrolytic conditions including current density for the manufacture of Zn-Ni alloy-electroplating steel sheet.
.circle.2 A multi-layered electroplating of Zn-Ni alloy is provided, the Ni content of each of the layer being different. The Ni content of a Zn-Ni alloy first layer deposited on a steel substrate is higher or lower than that of a Zn-Ni alloy layer to be placed thereon. See Japanese Patent Kokai 58-204196 and 58-6995.
.circle.3 A steel substrate is first flashed with Ni, Cu or the like to form a first ultra-thin layer, and then a predetermined Zn-Ni alloy is plated thereon.
These methods do in fact provide an improvement in the adhesion of Zn-Ni alloy electroplating layer upon impact. However, the level of improvement is still not satisfactory in light of present-day requirements. Furthermore, the resulting steel sheet does not meet requirements for resistance to chipping at low temperatures. i.e., the "low temperature chipping resistance", which is strongly desired in cold regions such as Canada and Northern U.S.A., where peeling of plating often occurs when gravel strikes against the exterior outer panels of automobiles at 100.about.250 km/hr at a temperature of -20.degree. C..about.40.degree. C. Therefore, the term "low temperature chipping resistance" means resistance to peeling by an electroplated layer when struck by flying grave at low temperatures.
Zinc alloy-plated steel sheet does not meet requirements regarding resistance to powdering, either. "Powdering" means the peeling-off of a Zinc alloy-plated layer in a powdery form. Powdering is undesirable because it results in spangle-like (star-shaped) defects on the surface of steel sheet for use in automobiles, electrical appliances, and the like, and because the pressing die must be frequently brushed off to remove the powder.
One of the inventors of the present invention proposed a method of improving adhesion of plating upon impact in Japanese patent application No. 61-51518. In that application, a preformed thin plating is dipped into a plating bath to dissove the plating, and then a Zn-Ni alloy electroplating is applied. This method is effective to improve the adhesion of plating upon impact. However, the resistance to powdering during press forming and the corrosion resistance after press forming are still not completely satisfactory.
Furthermore, in accordance with the flash-plating method described in .circle.3 , not only the above-mentined problems but also the occurrence of red rusting after coating are inevitable.
As already mentioned, adhesion upon impact is the adhesion which can keep the plating adhesive to the substrate even when pebbles hit against a steel sheet panel with a coating at a speed of 100.about.250 km/hr at low temperatures, such as -20.degree. C..about.--40.degree. C. This may also be called the adhesion upon impact dynamic deformation. On the other hand, powdering occurs during press forming and is due to bending and shearing stresses during forming and sliding of a sheet under high pressure against the press die. The resistance to powdering also depends on the adhesion of the electroplating layer to the steel substrate. Therefore, although improvements in impact adhesion and resistance to powdering are required for an electroplated layer, they should be distinguished from each other with respect to not only the shape of the peeled-off pieces from the surface of the sheet but also the mechanism by which they occur. When an impact stress is applied, for example, the peeled-off pieces are in the shape of sliced fine flakes. Thus, different measures are apparently necessary for improving adhesion upon impact and resistance to powdering.